This application relates to a method for end-capping polycarbonate resins and to end-capping compositions useful in such a method.
Polycarbonates prepared by the reaction of a dihydric phenol (such as bisphenol A, xe2x80x9cBPAxe2x80x9d) and a diaryl carbonate (such as diphenyl carbonate, xe2x80x9cDPCxe2x80x9d) in a melt transesterification process generally contain significant levels of uncapped chains (7-50%) as compared to interfacially prepared polycarbonates. These uncapped chains can have a significant impact on the resulting properties of polycarbonate, and it is therefore desirable in many instances to include an end-capping agent with a higher capping efficiency than DPC during or after the polymerization reaction which terminates the uncapped chains.
Known end-capping reagents are frequently carbonate or ester derivatives of phenol or even monohydric phenols themselves. EP 360578 discloses the use of sequestering agents to sequester hydroxy terminal groups to improve the following properties of melt polycarbonate: heat resistance, water resistance (resistance to hydrolysis), and color tone and tensile strength even after immersion in boiling water. In particular, the Examples of the ""578 patent teach the use of para-cumylphenol, 2-carbomethoxy-5-tert-butylphenyl phenyl carbonate, 2-carbobutoxy-5-tert-butylphenyl phenyl carbonate, p-cumylphenyl phenyl carbonate, p-cumylphenyl phenyl carbonate, and diphenyl carbonate as sequestering agents which are added to the initial monomer mix vessel of a continuous reactor system. Unfortunately the addition of excess carbonate or especially monohydric phenols to the initial monomer mix stage of the melt or transesterification process limits the reaction kinetics and thus achievable molar mass and end-cap level. U.S. Pat. No. 4,680,372 discloses the use of phenyl benzoate as an end-capping reagent to terminate polymers formed by melt polymerization of a bisphenol and an aromatic dicarboxylic acid such as terephthalic acid and/or isophthalic acid. U.S. Pat. No. 4,886,875 describes preparation of polyarylate compositions using diaryl carbonates, polyarylcarbonate oligomers or polyarylcarbonate polymers as end-capping agents. In particular, the Examples of the ""875 patent teach the use of diphenyl carbonate or highly endcapped polycarbonate oligomers as end-capping agents. Unfortunately these end-capping reagents all yield the byproduct phenol, which then rapidly re-equilibrates with the polycarbonate to limit the achievable molar mass and end-cap level. Long reaction and devolatization times are required to counteract this effect.
Therefore known end-capping reagents are frequently also carbonate or ester derivatives of electronegatively-ortho-substituted phenols which are more reactive than DPC. U.S. Pat. No. 4,310,656 describes the transesterification of bis(ortho-haloaryl)carbonates, haloaryl aryl carbonates, and a dihydric phenol, and indicates that controlled aryl end-capping is achieved. U.S. Pat. No. 4,363,905 describes the transesterification of bis(ortho-nitroaryl)carbonates, nitro aryl aryl carbonates and a dihydric phenol, and indicates that controlled aryl end-capping is achieved. It should be noted though that both bis(ortho-haloaryl)carbonates and bis(ortho-nitroaryl)carbonates have quite different properties than DPC. Thus the replacement of DPC by these compounds requires considerably different equipment and operating conditions than typically found in melt polycarbonate production. In addition the use of these compounds results in the production of colored or potentially toxic or explosive byproducts or ones that produce gaseous products containing chlorine upon combustion. Thus from product quality (transparency), handling, and environmental considerations there is a demand for the use of carbonates that are free from chlorine and nitro-activating groups. U.S. Pat. No. 4,661,567 describes the use of vinylene carbonate derivatives as end-capping agents which are added to preformed polycarbonates to terminate the polymers.
U.S. Pat. No. 5,696,222 describes a process for production of a terminally-blocked polycarbonate by melt transesterification of a dihydric phenol and a diaryl carbonate in the presence of an asymmetric substituted phenol ester or carbonate as an end-capping agent, and in particular end-capping agents which are salicylic acid derivatives. European Patent Publication No. 0 980 861 discloses an improved method for making such derivatives. These end-capping agents are derived from one salicylate (activated) and one non-activated phenol. While such end-capping agents are effective, they are not without their drawbacks. Specifically, such asymmetric carbonates require two separate steps for their preparation (generation of a chloroformate from one of the phenols, followed by condensation with the second phenol). This two step process adds significantly to the cost of the end-capping agent. An additional deficiency of this method is that the asymmetric mixed carbonates prepared in this way are often contaminated with traces of nitrogen- and halogen-containing impurities and with symmetrical carbonates derived from one or both of the phenols used in the reaction. As a consequence, in order to obtain materials of suitable quality for polymerization, purification is often both essential and difficult.
The ""222 patent also describes the use of di-activated carbonates, for example derived from two salicylates to either couple polycarbonate chains to increase molecular weight, or to cap phenolic hydroxyl end groups. This method suffers from the fact that only salicylate (activated) end groups can be incorporated. These salicylate end groups are different from the conventional phenol or alkyl-substituted phenol end groups typically found in commercial polycarbonate resins. In addition, capping is accompanied by coupling so that it is difficult to only cap without increasing molecular weight, or to systematically vary the endcap level without varying also the polycarbonate molecular weight. Indeed, while increasing levels of conventional end-capping agents tend to decrease molecular weight, in the case of the di-activated species there are opposing tendencies where the end-capping function reduces molecular weight while the coupling function tends to increase molecular weight, making the characteristics of the product difficult to predict or control.
U.S. Pat. No. 6,300,459 discloses increasing the molecular weight of a polycarbonate by reaction with a salicylic acid ester derivative such as bis(methyl salicyl) carbonate.
It has now been found that end-capping reagents which comprise a mixture of different species, including at least:
(a) a symmetrical activated aromatic carbonate, and
(b) an optionally substituted phenol provide effective end-capping of polycarbonate resins and can provide end-capped polycarbonates with controllable amounts of functionalized end-caps and with desirable physical properties. Thus, the present invention provides an end-capping reagent, and a method for preparing an end-capped polycarbonate resin using the reagent. In accordance with an embodiment of the method of the invention a mixture comprising a polycarbonate having free hydroxyl-end groups and an end-capping reagent is processed in a melt transesterification reaction to produce a polycarbonate resin. The carbonates and phenols of the end-capping reagent reacts with at least some of the free hydroxyl end-groups of the polycarbonate to produce an end-capped polycarbonate resin.
This application relates to the use of an end-capping reagent that is a mixture of different species, including at least:
(a) one species of symmetrical activated aromatic carbonate, and
(b) one species of optionally-substituted phenol.
This invention further relates to a method of making polycarbonate resin which uses an end-capping reagent of this type.
As used in the specification and claims of this application, the term xe2x80x9csymmetrical activated aromatic carbonatexe2x80x9d refers to compounds containing two phenolic groups linked through a carbonate bridge, with each phenol group being substituted with the same electronegative and therefore activating substituent at the ortho position. Many of these symmetrical activated aromatic carbonates can be represented by the general formula: 
wherein R is the electronegative substituent. Preferred electronegative substituents are carbonyl-containing groups, nitro groups, and halo groups. Symmetrical activated aromatic carbonates of this type may be synthesized by the reaction of an appropriate ortho-substituted phenol with phosgene.
Symmetrical activated aromatic carbonates for use in compositions and methods in accordance with the invention may also be made either by reaction of two equivalents of an appropriate xe2x80x9cactivatedxe2x80x9d or ortho-substituted phenyl chloroformate with a bisphenol, such as bisphenol A, or by reaction of a bischloroformate with two equivalents of an xe2x80x9cactivatedxe2x80x9d or appropriate ortho-substituted phenol. Where bisphenol A is used, the resulting composition has the following general formula, 
wherein R is as defined above.
Specific examples of symmetrical activated aromatic carbonates which may be used in the compositions and methods of the invention are summarized in Table 1.
As used in the specification and claims of this application, the term xe2x80x9coptionally substituted phenolxe2x80x9d refers to compounds containing one phenolic group. The phenol groups may be substituted or unsubstituted, provided that they do not include electronegative and therefore activating substituents at the ortho position. Many of these optionally substituted phenols can be represented by the general formula: 
wherein each R2 is independently H or some other optional substituent and n is an integer between 1 and 5. Suitable substituents which enhance the properties of the end-capped polycarbonate ultimately produced include C1-C36 alkyl (linear or branched chains), C1-C36 alkoxy (linear or branched chains), aryl (an arene group bonded directly to the phenol), aryloxy (an aryloxy group bonded directly to the phenol), arylalkyl (an arene tethered to the phenol by a linear or branched, C1-C36 alkyl), or arylalkoxy (an aryloxy tethered to the phenol by a linear or branched, C1-C36 alkyl). Specific non-limiting examples of optionally substituted phenol which may be used in the compositions and methods of the invention include phenol, p-cumylphenol, tert-butylphenol, octylphenol, nonylphenol, dodecylphenol, 3-pentadecylphenol, and octadecylphenol.
The mixture of the symmetrical activated aromatic carbonate and optionally substituted phenol may be preformed prior to addition to the melt transesterification reaction, or may be generated in situ by separate addition of the carbonate and phenolic species to the melt transesterification. This mixture, whether preformed or generated in situ, is referred to herein as the xe2x80x9cend-capping reagent.xe2x80x9d
The relative amounts of the carbonate and phenol in the end-capping reagent can be varied depending on the product characteristics desired. Higher amounts of activated carbonate will increase the degree of coupling obtained, while higher relative amounts of the optionally substituted phenol can be used to enhance the incorporation of a particular functional group or to limit the final molecular weight. In general, the mole ratio of activated carbonate to optionally substituted phenol will suitably range from 10:90 to 90:10.
Although it can increase the cost without substantial benefit in the composition, the end-capping reagent of the invention can include asymmetrical activated carbonates. For example, the end-capping reagent may contain 25 mol % or less of an asymmetrical activated carbonate of the formula: 
wherein R is an electronegative substituent and R2 is hydrogen, C1 to C36 alkyl, C1 to C36 alkoxy, C6 to C36 aryl, C7 to C36 aralkyl, or C7 to C36 aralkoxy. The endcapping-reagent may similarly include asymmetrical non-activated carbonates, for example 25 mol % or less of a symmetrical non-activated aromatic carbonate of the formula: 
wherein R2 is hydrogen, C1-C36 alkyl, C1-C36 alkoxy group, C6-C36. aryl, C7-C36 aralkyl, or C7-C36 aralkyloxy and n is 1-5.
In a specific embodiment, the end-capping reagent is preformed and consists essentially of one or more species of symmetrical activated aromatic carbonate, one or more species of optionally-substituted phenol and reaction products thereof. This preformed end-capping reagent may be equilibrated in a molten state prior to addition to a pre-formed polycarbonate, but such equilibration is not required. The molten end-capping reagent may be neat, or it may include appropriate solvents (such as PETS, or a mixture of toluene and acetone) to aid in handling. When the components of the end-capping reagent are added separately, they may be added as pellets, as a neat melt, or in an appropriate solvent.
When an equilibration step is included, it may be a pre-equilibration (preferably) or an in situ equilibration and may occur in the presence or absence of basic catalyst. It is particularly preferred to conduct the equilibration under conditions whereby transesterification of the optionally-substituted phenol by the symmetrical activated aromatic carbonate occurs. For example octadecylphenol and bis(methyl salicyl) carbonate in a molar ratio of 2:1 can be equilibrated to form bis(octadecylphenyl) carbonate and methyl salicylate, with the former being in solution in the latter when in the liquid state. This solution may be used for end-capping without separation of the two compounds.
Lower molar ratios than 2:1, most often from about 1:1 to 2:1, can be employed, whereby an asymmetric carbonate will be formed, at least formally. The use of end-capping reagents containing such asymmetric carbonates is also within the scope of the invention. Molar ratios greater than 2:1 will result in the presence of unreacted optionally-substituted phenol, and are not preferred.
The components used in the end-capping reagent and the relative amounts of the symmetrical activated aromatic carbonate and the optionally-substituted phenol in the end-capping reagent can be varied depending on the product characteristics desired. In general, using the end-capping reagent of the invention it is possible to obtain polycarbonate with some or all of the following advantageous characteristics: (1) lower free-hydroxyl endgroups resulting in improved thermal stability, lower water up-take, improved hydrolytic stability, and improved static performance compared to that of the polycarbonate that would result if no end-capping reagent were added; (2) equal or higher melt flow (or equal or lower melt viscosity) and improved shear-thinning rheological properties as compared to an unfunctionalized phenyl endcapped polycarbonate of similar molecular weight and polydispersity; (3) higher ductility compared to an unfunctionalized phenyl endcapped polycarbonate of similar molecular weight; and (4) lower glass transition temperature as a function of the functionalized phenyl endcap, as compared to an unfunctionalized phenyl endcapped polycarbonate of similar molecular weight. For example, glass transition temperatures in the range of 125 to 150xc2x0 C. can be obtained.
Preparation of the end-capping reagent In one embodiment of the invention, the symmetrical activated carbonates are prepared using a method in which two equivalents of a substituted phenol is reacted with one equivalent of phosgene in an interfacial reaction using water and a chlorinated solvent such as methylene chloride and in the presence of a base such as sodium hydroxide to neutralize the liberated HCl. Additional catalysts may be employed in this reaction to facilitate the condensation reaction. In one embodiment, the condensation catalyst is triethyl amine, quaternary alkyl ammonium salt, or mixtures thereof. After completion of the condensation reaction, the organic product phase is washed with aqueous acid and then with water until the washings are neutral. The organic solvent may be removed by distillation and the end-capper is crystallized or distilled and recovered.
End-capping Reaction in the Polycarbonate Production Process: The end-capping reagent of the present invention is used to react with at least some of the free hydroxyl end-groups of the polycarbonate to produce an end-capped polycarbonate resin. The phenols having electron-withdrawing ortho-substituents generated in the capping reaction are less reactive than phenol in backbiting reactions, which lead to molecular weight degradation of the polycarbonate. Therefore, the by-product phenols are removed from the terminal-blocked polycarbonate by distillation to the over-head system using conventional means (i.e., freeze traps using chilled water as a coolant) where they can be condensed to expedite the end-capping reaction at high yields.
It should be noted that the end-capped polycarbonate may still contain small amounts of any unrecovered phenols, any unreacted end-capping reagent along with by-products of any side reactions to the end-capping reactions, e.g. terminal 2-(alkoxycarbonyl)phenyl groups and the like. In one embodiment, the end-capped polycarbonate contains about less than 500 ppm of phenols having electron-withdrawing ortho-substituents and about 500 ppm of unreacted terminal blocking agent of the present invention. In another embodiment, the end-capped polycarbonate contains about 2,500 ppm or less, preferably 1000 ppm or less of terminal 2-(alkoxycarbonyl)phenyl groups, such as terminal 2-methoxycarbonyl)phenyl groups.
In one embodiment, the phenols having electron-withdrawing ortho-substituents by-product of the following formula is recovered from the overhead system and reused to prepare new end-capping reagents. 
In accordance with the method of the invention, end-capping reagent containing activated and non-activated carbonates is combined with a preformed polycarbonate polymer having free hydroxyl end groups. The preformed polycarbonate polymer may be any type of polycarbonate, and can be formed by either melt transesterification or an interfacial process, although most commonly the preformed polycarbonate polymer would be formed from the melt transesterification process.
Melt Polycarbonate Process The process of the present invention is a melt or transesterification process. The production of polycarbonates by transesterification is well-known in the art and described, for example, in Organic Polymer Chemistry by K. J. Saunders, 1973, Chapman and Hall Ltd., as well as in a number of U.S. patents, including U.S. Pat. Nos. 3,442,854; 5,026,817; 5,097,002; 5,142,018; 5,151,491; and 5,340,905.
In the melt process, polycarbonate is produced by the melt polycondensation of aromatic dihydroxy compounds (A) and carbonic acid diesters (B). The reaction can be carried out by either a batch mode or a continuous mode. The apparatus in which the reaction is carried out can be any suitable type of tank, tube, or column. The continuous processes usually involve the use of one or more CSTR""s and one or more finishing reactors.
Examples of the aromatic dihydroxy compounds (A) include bis(hydroxyaryl) alkanes such as bis(4-hydroxyphenyl)methane; 1,1-bis(4-hydroxyphenyl)ethane; 2,2-bis(4-hydroxyphenyl)propane (also known as bisphenol A); 2,2-bis(4-hydroxyphenyl)butane; 2,2-bis(4-hydroxyphenyl)octane; bis(4-hydroxyphenyl)phenylmethane; 2,2-bis(4-hydroxy-1-methylphenyl)propane; 1,1-bis(4-hydroxy-t-butylphenyl) propane; and 2,2-bis(4-hydroxy-3-bromophenyl)propane; bis(hydroxyaryl)cycloalkanes such as 1,1-(4-hydroxyphenyl)cyclopentane and 1,1-bis(4-hydroxyphenyl)cyclohexane; dihydroxyaryl ethers such as 4,4xe2x80x2-dihydroxydiphenyl ether and 4,4xe2x80x2dihydroxy-3,3xe2x80x2-dimethylphenyl ether; dihydroxydiaryl sulfides such as 4,4xe2x80x2-dihydroxydiphenyl sulfide and 4,4xe2x80x2-dihydroxy-3,3xe2x80x2-dimethyldiphenyl sulfide; dihydroxydiaryl sulfoxides such as 4,4xe2x80x2-dihydroxydiphenyl sulfoxide and 4,4xe2x80x2-dihydroxy-3,3xe2x80x2-dimethyldiphenyl sulfoxide; and dihydroxydiaryl sulfones such as 4,4xe2x80x2-dihydroxydiphenyl sulfone and 4,4xe2x80x2-dihydroxy-3,3xe2x80x2-dimethyldiphenyl sulfone. In one embodiment, the aromatic dihydroxy compound is bisphenol A (BPA).
Examples of the carbonic acid diesters (B) include diphenyl carbonate; ditolyl carbonate; bis(chlorophenyl)carbonate; m-cresyl carbonnate; and dinaphthyl carbonate. In one embodiment of an industrial process, diphenyl carbonate (DPC) is used.
The carbonic diester component may also contain a minor amount, e.g., up to about 50 mole % of a dicarboxylic acid or its ester, such as terephthalic acid or diphenyl isophthalate, to prepare polyesterpolycarbonates.
In preparing the polycarbonates, usually about 1.0 mole to about 1.30 moles of carbonic diester are utilized for every 1 mole of the aromatic dihydroxy compound. In one embodiment, about 1.01 moles to about 1.20 moles of the carbonic diester is utilized. This process can result in the formation of polycarbonate with a content of terminal alkylphenyl groups of about 0.25% or more, preferably 0.5 mole % or more.
Optional Terminators/End-capping Agents. In one embodiment of the melt process, additional/optional terminators or end-capping agents of the prior art may also be used. Examples of terminators include phenol, p-tert-butylphenol, p-cumylphenol, octylphenol, nonylphenol and other endcapping agents well-known in the art.
Optional Branching Agents. In one embodiment of the process of the present invention, branching agents are used as needed. Branching agents are well-known and may comprise polyfunctional organic compounds containing at least three functional groups, which may be hydroxyl, carboxyl, carboxylic anhydride, and mixtures thereof. Specific examples include trimellitic anhydride, tris(p-hydroxyphenyl)ethane, isatin-bisphenol, trisphenol TC (1,3,5-tris((p-hydroxyphenyl)isopropyl)benzene), trisphenol PA (4(4(1,1-bis(p-hydroxyphenyl)-ethyl) alpha,alpha-dimethyl benzyl)phenol, and benzophenone tetracarboxylic acid.
Optional catalysts. The polycarbonate synthesis may be conducted in the presence of a catalyst to promote the transesterification reaction. Examples include alkali metals and alkaline earth metals by themselves or as oxides, hydroxides, amide compounds, alcoholates, and phenolates, basic metal oxides such as ZnO, PbO, and Sb2O3, organotitanium compounds, soluble manganese compounds, nitrogen-containing basic compounds and acetates of calcium, magnesium, zinc, lead, tin, manganese, cadmium, and cobalt, and compound catalyst systems such as a nitrogen-containing basic compound and a boron compound, a nitrogen-containing basic compound and an alkali (alkaline earth) metal compound, and a nitrogen-containing basic compound, an alkali (alkaline earth) metal compound, and a boron compound.
In one embodiment of the invention, the transesterification catalyst is a quaternary ammonium compound or a quaternary phosphonium compound. Non-limiting examples of these compounds include tetramethyl ammonium hydroxide, tetramethyl ammonium acetate, tetramethyl ammonium fluoride, tetramethyl ammonium tetraphenyl borate, tetraphenyl phosphonium fluoride, tetraphenyl phosphonium tetraphenyl borate, tetrabutyl phosphonium hydroxide, tetrabutyl phosphonium acetate and dimethyl diphenyl ammonium hydroxide.
The above-mentioned catalysts may each be used by themselves, or, depending on the intended use, two or more types may be used in combination. When more than one catalyst is employed, each may be incorporated into the melt at a different stage of the reaction. In one embodiment of the invention, part or all of one catalyst is added together with the end-capping reagent.
The appropriate level of catalyst will depend in part on how many catalysts are being employed, e.g., one or two. In general, the total amount of catalyst is usually in the range of about 1xc3x9710xe2x88x928 to about 1.0 mole per mole of the dihydroxy compound. In one embodiment, the level is in the range of about 1xc3x9710xe2x88x925 to about 5xc3x9710xe2x88x922 mole per mole of dihydroxy compound. When more than one catalyst is employed, each may be incorporated into the melt at a different stage of the reaction.
Other optional components in the polycarbonate In the present invention, the polycarbonate obtained may further contain at least one of a heat stabilizer, an ultraviolet absorbent, a mold releasing agent, a colorant, an anti-static agent, a lubricant, an anti-fogging agent, a natural oil, a synthetic oil, a wax, an organic filler and an inorganic filler, which are generally used in the art.
Adding the End-Capping Agent to the Melt Process
The method of adding the end-capping agent to polycarbonate is not specially limited. For example, the end-capping agent may be added to the polycarbonate as a reaction product in a batch reactor or a continuous reactor system. The end-capping reagent may be suitably added to molten polycarbonate and mixed in a static mixer. The end-capping reagent can also be combined with the polycarbonate in a reactor system which includes, in series, one or more reactors, one or more polymerizers and an extruder through which the polycarbonate is transported. In this case, the end-capping reagent is suitably added before one of the polymerizers, but after the polycarbonate has passed through at least the initial reactors to build up the desired molecular weight. In one embodiment, the end-capping agent is added to the melt polycarbonate just before or after a later reactor, i.e., a polymerizer, in a continuous reactor system. In a second embodiment, the end-capping agent is by reactive extrusion after the last polymerizer in the continuous reactor system. In a third embodiment, it is added between the 1st and 2nd polymerizer in a continuous reactor system. In yet another embodiment, the end-capping agent is added between the 2nd reactor and the 1st polymerizer.
The amount of end-capping reagent appropriately utilized can be quantified with reference to the amount of free hydroxyl end groups in the pre-formed polycarbonate polymer. In general, the mole ratio of total carbonate to free hydroxyl end groups is within the range of from 0.5 to 3.0, depending on the level of end-capping desired.
In some reactor systems and extruders, it may be difficult to reach the desired equilibrium ratio of introduced or incorporated non-activated and activated end-groups (the non-activated end groups are preferentially present at equilibrium) due to poor mixing, short residence time, or rapid volatilization of one component. In such a case the ratio of introduced non-activated to activated end groups can be favorably increased by melt mixing the activated carbonate and optionally-substituted phenol together in the presence of a small amount of basic catalyst (such as tetramethylammonium hydroxide) to produce a scrambled product consisting of a pre-equilibrated or statistical mixture of reactants. The invention will now be further described with reference to the following non-limiting examples.
Starting Material Polycarbonate In all examples, either starting polycarbonate grade A, B, C, or D was used. The starting materials were prepared by a melt process in a continuous reactor system with the following properties:
In the Examples, the following measurements were made.
a) Molecular weight: Mw and Mn were measured by GPC analysis of 1 mg/ml polymer solutions in methylene chloride versus polystyrene standards. Unless otherwise stated the measured polycarbonate Mw and Mn values were then corrected for the difference in retention volume between polycarbonate and polystyrene standards.
b) Free-OH content was measured by UV/Visible analysis of the complexes formed from the polymer with TiCl4 in methylene chloride solution. In some cases the Free OH content was measured by direct infrared or 31P NMR methods.
End-cap levels were calculated from the free OH content and Mn values.
Incorporation levels of specific end groups were determined by NMR.
The glass transition temperature of some end-capped polycarbonates was measured by differential scanning calorimetry.